Apparatus for the plasma treatment of substrates in a plasma discharge which is excited by high frequency and occurs between two electrodes to which power is supplied by a high frequency source. The first electrode is configured as a hollow electrode, and the second one is an electrode which carries a substrate and is disposed upstream of the hollow chamber of the first electrode which it also passes. The hollow electrode is surrounded by a dark space shield and has an edge pointing in direction toward the second electrode and also has projections between the edge which are on the same potential as the second electrode.
An apparatus for the plasma treatment of substrates by means of high frequencies usually has two electrodes one of which is formed by the vacuum chamber and/or the substrate holder which are both made of a metallic material. It is known that a plasma of a predominantly negative voltage develops at the one electrode which, with respect to those surfaces that act as counter electrodes, have the smaller active surface. The negatively biased electrode is hence usually referred to as "cathode."
When, on the one hand, a plate-like substrate holder is used which has the potential as the vacuum chamber thus forming the electrode and, on the other hand, when a plate-like electrode is used which is opposite the substrate holder, the negative potential will form due to the necessarily given surface proportions on the latter electrode. Material (=target) located on this electrode (=cathode) is thus sputtered and deposited on the substrate. If the substrates are to be etched in such an apparatus, substrates and target must be exchanged correspondingly.
When a direct voltage is used, then the polarity of the electrodes is definitely prescribed by its connection to the respective pole of the direct current source. From DE OS 21 15 590, for example, it is known to use a hollow cathode which has an edge protruding in direction toward the substrate carrier in order to improve the uniformity of the layer thickness distribution. However, when direct current is used, this protruding edge does not bring about a reversal of the polarity.
DE 22 41 229 discloses an apparatus where a high frequency is used to accomplish the following; due to its edge, the hollow electrode relative to the surface of other electrode (of the substrate holder) which directly faces the hollow chamber, has the larger surface. It thus assumes the function of an anode so that there is reversal of the relations, so to speak, with respect to the bias voltage. This can also be explained in less complicated terms as follows: when there is a sufficiently small space between this other electrode and the edge of the hollow electrode (gap S), the edge of the hollow electrode defines a discharge chamber relative to which those metallic parts of the apparatus, which are on the outside, no longer execute any electrode functions. The proportions are exclusively determined, on the one hand, by those surface parts of the hollow electrode which face one another and, on the other hand, by the substrate holder. This is referred to as the "edge effect."
However, when this known principle of the solution is applied in an apparatus with a continuous substrate transport, some problems will occur: When the substrate holder with the substrate is moved away, at least the metallic base plate of the vacuum chamber will act as a counter electrode. It usually has a larger surface than the hollow electrode and is connected to ground. In this case, a negative voltage, in complete reversal of the potentials, will be generated on the hollow electrode, now acting as a cathode.
This shows that the size of the gap between the edge of the hollow electrode, on the one hand, and the substrate or substrate holder, on the other hand are of essential importance to the distribution of the potential in the area of the chamber filled by plasma. Since this gap constitutes the connection of the plasma to its surroundings, the size of the gap will decide whether the hollow electrode will act as an anode or a cathode of the arrangement. Experiments have shown that when the gap is enlarged by an amount of more than 2 mm, the effect increases with the consequence that a negative bias voltage is generated on the hollow electrode instead on the substrate. Such enlarged gaps, however, are practically unavoidable in continuous apparatus where the substrates are disposed on movable substrate holders. This applies especially when such substrate holders, while spaced apart, pass the opening of the hollow electrode. The, the width of this gap cannot be freely selected since this would require an extremely precise control of the substrate holders. The substrates, which can be on the surface of the holder, modify the gap width while they traverse the apparatus unless expensive provisions are made to use a different substrate holder for each substrate form and size into which the substrates are then lowered so as to be even with the surface of the substrate holder. This greatly reduces the range of application of the known principle.
Further, an apparatus of the aforesaid kind is known (DE OS 36 06 959 A1) for the plasma treatment of substrates in a plasma discharge excited by high frequency and located between two electrodes to which power is also supplied by a high frequency source. The first electrode is configured as a hollow electrode, and the second one which carries a substrate is situated opposite of the first electrode and passes it. In this known arrangement, the hollow electrode is surrounded by a dark space shield and by an edge pointing in direction of the second electrode. Further, within the edge there are projections which are approximately on the same potential as the first electrode. Since this known arrangement does not allow a decoupling between radio frequency power and direct voltage power of substrate and plasma (selfbias=substrate bias voltage), the selfbias can only be influenced by modifying the radio frequency. Also, when the power of the radio frequency is reduced, the deposition rate also decreases which often impairs the quality of the substrate.